A network model of a broadband access network is shown in FIG. 1.
As shown in FIG. 1, the broadband access network system generally includes: one or more user access devices, an Ethernet bridge device, a Broadband Remote Access Server (BRAS), an Authentication, Authorization and Accounting (AAA) Server, etc. The user access devices may be, for example, a Digital Subscriber Line Access Multiplexer (DSLAM), a Local Area Network (LAN) switch (Ethernet bridge) etc. The Ethernet bridge device may be, e.g., an ATM exchange, an Ethernet bridge, etc.
The user access device implements the function of subscriber line tandem. The Ethernet bridge device couples the user access device to the BRAS. The BRAS terminates the user access link layer and cooperates with the AAA server to implement user access authentication and accounting. The user access device and the Ethernet bridge device operate in a layer-2 mode, only handle the link layer information in a data packet, and forward the packet according to the link layer address information.
As the cost of Ethernet components decreases greatly and the exchange capacity and performance of L2/L3 (layer-2/layer-3) exchanges improve ceaselessly, a newly-built broadband access network usually employs Ethernet-based technologies, in other words, the user access device realizes the uplink via an Ethernet interface such as FE/GE and an access convergence network is constructed by employing an Ethernet bridge device. To guarantee user access security and QoS, one or more exclusive VLANs are usually occupied by a user access device
At present, the access convergence network constructed via Ethernet bridge devices may have various network topology structures, for example, tree network topology, ring network topology, etc.
In FIG. 2, SW1 to SW6 are ordinary Ethernet bridge devices. The operational principle of an Ethernet bridge device is as follows: a Media Access Control (MAC) address table is generated by learning a port information associated with a MAC address; when a data packet is to be forwarded, the MAC address table is searched according to the destination MAC address in the data packet to obtain an output port corresponding to the destination MAC address; and then the data packet is sent from the output port. If no record in the MAC address table matches the destination MAC address during the search of the MAC address table, the data packet will be broadcasted, in other words, the data packet will be copied for each port of the Ethernet bridge device.
The Ethernet bridge device at the convergence point in the network is, for example, SW1 and SW2 as shown in FIG. 2. The space of the MAC address table thereof must be large enough, so that it may be guaranteed that the MAC addresses of all the users who access via the Ethernet bridge device can be learnt. As shown in FIG. 2, if there are 4000 users to access each of the DSLAMs, the MAC address table of SW1 must be able to support at least 8000 MAC addresses to be learnt, and the MAC address table of SW2 must be able to support at least 12000 MAC addresses to be learnt. If there is no enough space of the MAC address table, the MAC addresses of a part of the users may not be learnt. As a result, the data packets sent to the part of the users can only be broadcasted, so that the network bandwidth will be wasted and instability of the network will increase.
When an access convergence network is built, if a high-level Ethernet bridge device with specific advantages, for example, an Ethernet bridge device including a MAC address table with large space is employed, the networking cost will become very high. If a low-level Ethernet bridge device with lower price and MAC address space less than 8 k is employed for networking, the scale of the network will be limited, so that the networking cost will finally increase due to the limitation of the scale of the network.